I am 23 years old and have been having chest pain beneath and on my left breast (and on my left arm) for almost 3 months what can it be?

Not heart: The likelihood of heart disease at your age is pretty close to zero. Statistically the most likely cause is a muscular one. Nerve root impingement in your neck (like from a cervical disc herniation) could also account for your sx. A physical exam and some judicious imaging studies would have a good chance of successfully determining the cause.

Several things: There are several things that can cause your symptoms, and a lot depends on your history (what makes the pain better or worse, quality of the pain, etc) and physical exam - any lumps, point tenderness, associated symptoms like shortness of breath can lead a doctor to look at the heart, lungs. ribcage, breast, or nerves depending on your history. Please see your doc, since it has been nearly 3 mo.